Production of light metal forgings



Patented Mar. 1.8, 1941 UNITED STATES PATENT) orr es 2,235,415 PRODUCTION OF LIGHT METAL FORGINGS No Drawing. Application May 27 1938, Serial No.

In Germany June 7, 193? 5 Claims. (01. 14811.5)

This invention relates to a process for the production of improved forgings from high percentage magnesium alloys.

In the production of forgings from high percentage magnesium alloys, which are subsequently intended to undergo improving treatment by precipitation of constituents, held in supersaturated solid solution, it has hitherto been 'usual to subject the forged workpieces, after cooling, e. g. to room temperature, to a solution heat treatment, followed by an artificially retarded cooling orafter intermediate quenching, for example in water-by an annealing treatment at temperatures below the boundary line between the a and oz-l-fi fields in the phase diagram of the alloy system in question. When operating in this manner, it has however been observed, particularly in the case of forgings of large dimensions that, owing to the strongly "marked tendency of magnesium and high percentage magnesium, alloys rto recrystallise, crystals of large dimensions are formed during the heat treatment. This formation of large crystals adversely affects the mechanical strength values of the workpieces.

It has now been ascertained that the foregoing drawback can be avoided'by carrying out the forging operation at temperatures within the solid solution range (these temperatures being generally somewhat higher than hitherto customary in the forging of magnesium base alloys) and then subjecting the workpieces inimmediate succession to the forging operation, i. e. immediately on their being taken from the forging dies, to quenching, whereupon they are annealed at such temperatures as are only just sufficient to effect the desired precipitation of the heterogeneous constituent. These temperatures are generally below 200 C. and their application thus does not result in a recrystallisation of the workpiece to any appreciable extent.

The forging and tempering temperatures to be employed in carrying out the invention are naturally somewhat dependent on the composition of the alloy to be worked. As far as the former are concerned, they can be taken directly from the corresponding phase diagram. In respect of the latter it is. known that precipitation of a heterogeneous constituent, while always requiring temperatures below the boundary line separating the a and the a-l-fl field of the phase diagram, is dependent on the specific temperature employed and the duration of annealing, the temperature required for producing segregation beingthe lower the longer the heat treatmentis extended. On the other-hand, with magnesium base alloys recrystallisation does, in any case, not take place at temperatures below about 200. C., and generally becomes perceptible only in the 5 temperature range between about 230 and 250 C.: By suitably extending the duration of annealing, it is therefore always possible to produce precipitation of the hetero-geneous constituent held in supersaturated solid solution by the application of temperatures below about 200 C., i. e. such temperatures as will not result in a recrystallisation ofthe workpiece. For example, for alloys of the composition:

Per cent Aluminium 8-9 Zinc 0.2-0.6 Manganese 0.2 Magnesium I Balance 20 a forging temperature of from 320 to 400 C. is correct, while the annealing temperature should lie between 140 and 180 C. I

The invention of course presuppcses that the alloys, before being forged, are in the homogeneous state, this being generally brought about automatically by the heating of the alloys necessary for carrying out the forging operation.

Example Per cent Aluminium 8.5 Zinc 0.5 Manganese 0.2 40

was placed at a temperature of from 370 to 400 C. in a forging press with heated dies kept at a temperature of from 300 to 350 C., and there first converted into an intermediate shape suitable for the final forging. After the workpiece had been reheated to temperatures of from 340 to 360 C., the actual propeller blank was forged from this intermediary shape, likewise in heated dies, in a hydraulic press. After the forging operation had been completed, the propeller blank, which now had a temperature of about 300 to 330 C. was removed from the mould and immediately quenched in water. This operation was followed by an annealing treatment in an oil magnesium base alloys, which comprises forging v bath at a temperature of C., for from 4 to 6 hours. The strength values found in the finished workpiece are as follows:

Tensile strength=32-35 kgs. per square mm. Elongation=10-14% Yield point=22-24 kgs. per square mm. whereas when the alloy was treated by the usual process hereinbefore referred to, the correspond ing values were v Tensile strength=28-32 kgs. per square mm. Elongation=8-12% Yield point=13-22 kgs. per square mm.

I claim:

1. A process for the production of forgings from precipitation hardenable high percentage the substantially homogeneous alloys at temperatures within the solid solution range, thereafter immediately quenching the workpieces, and finally annealing them under conditions ensuring the formation of a hardening precipitate while not being adapted to cause recrystallisation of the crystal structure of the workpieces.

2. A process for the production of forgings from precipitation hardenable high percentage magnesium base alloys, which comprises forging the substantially homogeneous alloys at temperatures within the solid solution range, thereafter immediately quenching the workpieces, and finally annealing them at temperatures below about 200 C. so as to cause precipitation of a hardening constituent.

3. A process for the production of forgings from magnesium base alloys containing between about 8 and about 9% of aluminium, which comprises forging the alloys at temperatures from about 320 and about 400 C., thereafter immediately quenching the workpieces, and finally annealing them at temperatures between about 140 and C. so as to cause precipitation of a hardening constituent.

4. A precipitation hardened forging producedabout 0.6% of zinc and about 0.2% of manganese,-

balance magnesium, by forging said alloy at temperatures within the solid solution range and thereafter restricting all reheating to less than the recrystallization temperature, said alloy being characterized by a fine-grained crystalline structure of substantially uniform grain-size.

HUBERT ALTWICKER. 

